The thiazolidinediones (TZDs) are powerful anti-diabetic drugs whose use in treating type 2 diabetes is limited by adverse side effects. The goal of this proposal is a biochemical investigation into a mechanism that separates the positive metabolic effects of TZDs from their side effects, making it possible to design a better class of agents. PPAR?, the molecular target of the TZDs, is a key regulator of systemic insulin sensitivity, adipogenesis, inflammation, and energy homeostasis. In adipose tissue, phosphorylation of PPAR? at serine 273 (S273) is observed shortly after the initiation of high fat diet feeding and increases with progressive obesity. This phosphorylation correlates with dysregulation of PPAR? target genes, such as decreased expression of the insulin-sensitizing hormone adiponectin. ERK is the primary kinase responsible for phosphorylating PPAR? S273, inhibitors of the MEK/ERK kinase pathway block PPAR? S273 phosphorylation. Surprisingly, MEK/ERK inhibitors had potent anti-diabetic effects in obese mice demonstrating markedly improved glucose homeostasis. Similarly, high-affinity ligands of PPAR??which lack the capacity to promote adipogenesis but still block S273 phosphorylation retain anti-diabetic effects. These two pharmacological interventions blocking PPAR? S273 phosphorylation both promote improved peripheral metabolic homeostasis like the TZDs while also appearing safer as they do not trigger the side effects associated with TZDs. Our hypothesis is that ERK-mediated phosphorylation of PPAR? in obesity and inflammation causes altered impaired glucose homeostasis by targeting adipose tissue transcriptional regulation. We will test this hypothesis using a novel genetically modified mouse where PPAR? cannot be phosphorylated on S273 (S273A). Our preliminary data suggest that blocking this phosphorylation is sufficient to improve insulin sensitivity in obesity. In this proposal we will utilize three approaches to understand the contribution of PPAR? phosphorylation to the pathogenesis of obesity. In Aim 1, we will interrogate the effects of PPAR? S273 phosphorylation on glucose homeostasis and the ability of PPAR? S273A mice to respond to PPAR? ligands. In Aim 2, we will investigate the relative contribution of blocking S273 phosphorylation in the immune system to adipose tissue inflammation and insulin resistance. In Aim 3, we will examine the genome-wide set of mRNA transcripts regulated and cis-regulatory elements bound to either wild-type PPAR? or phosphorylation independent S273A PPAR?. Understanding how obesity and inflammation modulates PPAR? by ERK- mediated phosphorylation will be important for the future design of new therapeutic molecules.